Question: how big is the universe?

Roberto Trotta answered on 13 Jun 2014:

The Universe itself might be infinite, ie it goes on for ever.

But the part of the Universe that we can see is limited, because light can only travel at a certain speed (300,000 km per second). This means that there are places in the Universe that we can never see, because they are too far away for light from there to ever reach us.

So while the entire Universe might well be infinite, the part that we can see is about 80 billion light year across — this is a HUGE distance! It means that it would take a beam of light 80 billion years to traverse it (at 300,000 km/s).

In km, that is 7 500 000 000 000 000 km (more or less!).

Now you might think that this sounds wrong: if the Universe is 14 billion year old (which it is!), then surely the largest distance we can see to is 14 billion light years (ie, the distance that light could have travelled during the lifetime of the Universe).

That is almost right, but it forgets that the Universe is actually expanding: so objects (eg distant galaxies) from which light left billion of years ago have been pushed even further away by the expansion of the Universe in the meantime. That’s why we can see further out that 14 billion light years.